Photovoltaic devices generally include stacked layers of photoelectric materials and electrodes that convert light energy into electrical energy. The photoelectric materials can include silicon-based materials, polymeric materials, organic materials, and the like. Photovoltaic devices that use organic materials as the active layer are termed organic photovoltaic devices (OPVs) and can have varying structures depending on efficiency characteristics, design characteristics, and so forth. One such type of OPV structure includes bulk heterojunction (BHJ) structures that utilize various morphologies of electron donor and electron acceptor materials that are mixed or blended together. The morphology of the electron donor and electron acceptor materials of the BHJ structures can influence the efficiency of the associated photovoltaic devices.
The discovery of bulk heterojunction (BHJ) structures has paved the road for high efficiency energy conversion by organic photovoltaic devices (OPVs). The morphology of BHJ films plays a critical role in charge generation, collection, or recombination in BHJ OPV devices. A theoretical BHJ film morphology can include a bicontinuous interdigitated donor and acceptor network with donor and acceptor nanodomains within the exciton diffusion lengths or the uncertain length of light for efficient charge generation and extraction. However, actual morphologies of most BHJ films are generally far from ideal due to the random mixing of donors and acceptors in solution. Regular BHJ films fabricated from a blended solution inevitably have many breaks and dead ends. Techniques such as thermal annealing, solvent annealing, and mixing additives have been shown to successfully improve the morphologies of some BHJ films by increasing crystallinity of the donors and acceptors and/or forming nanodomains of donors and acceptors with suitable domain size to facilitate exciton dissociation and charge generation. However, these techniques do not address connectivity of the donor and acceptor network and are only applicable to specific material systems.
Other BHJ structures can utilize pre-formed n-type nanorod/nanowire arrays followed by infiltration of semiconducting polymers, but such structures do not provide power conversion efficiencies (PCEs) comparable to those made of blended films, which can be limited by interface issues.
Another issue with regular BHJ films involves a mismatch between the photocurrent distribution and charge transport channel width in the vertical direction, such as having an electron current that is higher toward the cathode side and a hole current that is higher toward the anode side (e.g., which can be due to nonsymmetrical charge collection in OPVs), whereas the distribution of donors and acceptors in such BHJ films is generally uniform due to the uniform mixing of them in solution. This mismatch can cause a piling up of photogenerated charges, resulting in recombination, and can severely reduce device PCE when the photogenerated charge density is high (e.g., low bandgap active layers, strong illumination, or so forth), or when photogenerated charges cannot be efficiently extracted out of the active layer due to low carrier mobility or a thick active layer. Such morphologies can cause charge recombination in the BHJ films and at the metal/organic interface, where the charge recombination can involve geminate recombination or bimolecular recombination.
Another BHJ structure includes compositionally graded BHJ films, which are donor enriched at the anode and acceptor enriched at the cathode side, and can facilitate the charge extraction and reduce charge recombination. Such graded structures can provide a donor rich anode side and an acceptor rich cathode side to better match the distribution of current, which can reduce the piling up of electrons close to the cathode side and holes close to the anode side and reduce the leakage of photogenerated charges to the wrong electrodes, thus reducing bimolecular charge recombination. However, fabrication techniques of some compositionally graded BHJ structures can affect the efficiency of the structures, such as by imparting limited bicontinuous connectivity of donors and acceptors.
Accordingly, there is a need for improved graded BHJ structures and methods of forming the same.